(1) Field of the Invention
This invention relates to a cylinder block reinforcement construction in an engine of the half-skirt type in which the axis of a crank shaft of the engine is located at a lower end portion of the cylinder block.
(2) Description of the Prior Art
A cylinder block reinforcement construction for an engine of the deep-skirt type has been known, as for instance disclosed in Japanese Utility Model Publication No. 55-40360(1980), in which a reinforcement member is disposed between the cylinder block and the oil pan. The reinforcement member comprises a frame member clamped between the skirt portion of the cylinder block and a rail or rim portion of the oil pan, a plurality of transverse members formed integrally with the frame member and fastened by bolts to main bearing caps of the engine, and a longitudinal member integrally formed so as to connect the transverse members to each other. The reinforcement member functions to enhance the stiffness of the skirt portion of the cylinder block as well as of the portions surrounding the main bearings.
An application of the above-mentioned reinforcement member to an engine of the half-skirt type, by reducing the weight of the reinforcement member while maintaining a high degree of stiffness of the reinforcement member, may be contemplated. For instance, the reinforcement member may be formed in a thinner platelike overall configuration, and the portions thereof directly below the cylinders may be projected downward so as to avoid interference with the crankshaft, thus, making the reinforcement member assume a dishlike sectional shape.
If the space defined by the cylinder block and the oil pan is completely partitioned by the platelike reinforcement member into an upper portion on the crank chamber side and a lower portion on the oil pan side, the internal volume of the upper space portion on the crank chamber side is reduced, hindering the movement of gases in the crank chamber accompanying the reciprocation of the piston in each cylinder. Then, the back pressure on the piston is increased, and the attendant increase in resistance to the reciprocating motion of the piston causes a lowering of the thermal efficiency of the engine. It is therefore necessary to provide the platelike reinforcement member with bottom vent holes for commumication between the upper space portion on the crank chamber side and the lower space portion on the oil pan side, at portions of the reinforcement member directly below the cylinders.
Providing such bottom holes, however, leads to a lowering of the stiffness of the reinforcement member, and thus, it is desirable that the aperture area of each bottom hole be as small as possible. The increase in the back pressure on a given piston depends on the relationships of the reciprocating motion of such piston with those of the adjacent pistons. When the bottom holes directly below the cylinders are all equally sized, therefore, the back pressure on the piston in a certain cylinder will be higher.